Automatic Protection Switching (APS) is a means to provide redundancy on Synchronous Optical Network (SONET) equipment to guard against line failures.
There are three types of linear APS modes, namely, 1+1, 1:1 and 1:n. All three modes require that if any failures are detected in a working line, a switch to a protection line must be initiated in 10 msec and completed in 50 msec for a total of 60 msec.
In the linear APS 1+1 mode, for every working line, there is a corresponding redundant protection line. Traffic is carried by both the working and protection lines simultaneously.
In the linear APS 1:1 mode, for every working line, there is a corresponding redundant protection line. However, traffic is normally carried on the working line only. When a failure is detected in the working line, traffic is switched to the protection line.
In the linear APS 1:n mode, for every ‘n’ working lines, there is a corresponding redundant protection line. Traffic is normally carried on the working lines only. When a failure is detected in a working line, traffic is switched to the protection line.
Currently, APS is implemented electrically through a multiplexer (MUX)/buffer combination on a line card. The disadvantages inherent in the current technology are many. For example, the APS ports must be in predefined locations, typically adjacent to each other. Additionally, the current scheme does not work at high speed signal transmissions. More specifically, although repeating signals at low speed is tolerable, the scheme breaks down at high speeds due to impedance mismatch and capacitive loading.